The present invention relates to a thermosetting resin composition and also relates to a two-parts composite body consisting of a first part of a cured silicone rubber and a second part of a thermosetting resin composition integrally bonded without intervention of an adhesive layer. More particularly, the invention relates to a thermosetting resin composition capable of bonding to the surface of a silicone rubber body, upon which the resin composition is cured, to exhibit a high bonding strength even without the use of any adhesives.
It is a trend in recent years that two-parts composite bodies, which consist of a first part having rubbery elasticity as formed from a rubber having a Shore A hardness, for example, in the range from 30 to 80, and a second part made from a cured and rigid resin having a Shore D hardness of, for example, at least 40, integrally bonded to the first part, if possible, without use of an adhesive, are sometimes required in various applications. One of the typical examples of such two-parts composite bodies is a so-called push button switch covering member of a keyboard switch panel for various kinds of electronic instruments. A push button switch covering member in general has a structure of a continuum consisting of a plurality of unit push button switches each having a structure illustrated in FIG. 7 of the accompanying drawing by a vertical cross sectional view consisting of a base part 75, a riser part 74 and a keytop base 72 integrally molded from a rubbery material and a keytop 71 made from a rigid resinous material and bonded to the keytop base 72 while a movable contact point 76 made from an electroconductive rubbery material is bonded to the lower surface of the keytop base 72. When the keytop 71 is pushed down by the fingertip of an operator, the riser part 74 is bent and the movable contact point 76 is brought into contact with a pair of fixed contact points (not shown in the figure) on the circuit board below (not shown in the figure) on which the push button switch covering member is mounted at the base part 75, so as to close the electric circuit between the fixed contact points. When the pushing force to the keytop 71 is released, the riser part 74 regains the undepressed form by the resilience of the rubbery material so that the movable contact point 76 is pulled up apart from the fixed contact points to open the electric circuit.
The rubbery material forming the base part 75, riser part 74 and keytop base 72 is required to have a Shore A hardness of 80 or lower in order to ensure good working behavior in the above described pushing and releasing operations as well as to ensure good position adjustability in mounting of the covering member on a circuit board. Among various kinds of rubbery materials, silicone rubbers are preferable in most cases in respects of their excellent electric properties, precision moldability and durability in repeated bending and releasing movements.
On the other hand, the keytop 71 is made from a rigid thermosetting resin having, preferably, a Shore D hardness of 40 or higher. This is because, when the keytop 71 is molded from a rubbery material integrally with the other parts, the top surface of the keytop 71 more or less has tackiness inherent in rubbery materials so as to disturb smoothness of the button-pushing operation. In addition, the fatty excreta excreted from the finger tip of the operator are absorbed by and infiltrate the rubbery material from the top surface of the keytop 71 to cause discoloration and degradation in the properties, e.g., mechanical strengths, of the rubber-made keytop. When the movable contact point 76 below the keytop is contaminated with the fatty excreta by migration through the keytop 71 and keytop base 72, an increase is caused in the electric resistance of the electroconductive rubbery material forming the movable contact point 76 to decrease the reliability of switching.
It is of course that a piece for the keytop 72 is separately prepared from a thermosetting resin and the piece is bonded to the upper surface of the rubbery keytop base 72 by using an adhesive. This method, however, is not practical because of the increased compliancy in the manufacturing process of the push button switch covering members if not to mention the disadvantage caused by the unevenness in the adhesive bonding between a rubbery keytop base 72 and the rigid resinous keytop 71. The above described situations form the reason for the recent trend that the push button switch covering member is prepared in a two-parts composite body integrally molded without using an adhesive for adhesive bonding of the resinous keytop 71 to the rubbery keytop base 72.
In the prior art, various attempts and proposals have been made heretofore for a thermosetting resin which can be integrally bonded to the surface of a silicone rubber part by means of a chemical reaction forming chemical bonds without using an adhesive. A class of the thermosetting resin compositions of this type includes those disclosed in Japanese Patent Kokai 6-309988 which are high-hardness silicone resins including methyl silicone resins, phenyl silicone resins, dimethyl diphenyl silicone resins and the like. The other class of the thermosetting resin compositions includes those disclosed in Japanese Patent Kokai 6-60767 consisting of a polyfunctional compound of which the concentration of the unsaturated groups is at least 6.times.10.sup.-4 moles/ml admixed with an organic peroxide to be imparted with thermal curability. The above mentioned polyfunctional compound is exemplified by ethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate and the like.
Each of the above described prior art thermosetting resins chemically bondable to a silicone rubber has its own advantages and disadvantages. For example, the high-hardness silicone resins disclosed in Japanese Patent Kokai 6-309988 each have very high transparency comparable to poly(methyl methacrylate) resins and polycarbonate resins and are capable of giving a resin-made part with a total light transmissivity of 90% or even higher to enable preparation of colored parts in any bright colors. In addition, these resins exhibit a linear shrinkage in molding approximately equivalent to that of silicone rubbers so that the integrally molded twoparts body is free from the troubles of warping or distortion due to difference in the molding shrinkage between the two parts. On the other hand, the mechanical strength or, in particular, impact strength of these silicone resins is relatively low so that, when the thickness of the keytop 71 is small, for example, to be smaller than 2.3 mm in a push button switch covering member, cracks are sometimes formed in the keytop 71 in a mechanical shock test in which the covering member is dropped from a height of about 2 meters onto the floor covered with resin tiles so that carefulness is required in handling of such push button switch covering members.
In contrast thereto, the thermosetting resins disclosed in Japanese Patent Kokai 6-60767 generally have a relatively high mechanical strength as an inherency of the resin so as to clear the above mentioned mechanical shock test. Nevertheless, the application fields of these resins are limited because, since the resins have a Shore D hardness of 50 to 60, the resin surface is susceptible to scratch formation even by scratching with a finger nail or a pencil tip. In addition, these resins have another problem relating to the molding shrinkage usually exhibiting a linear shrinkage of as large as 7 to 8% as is explained by making reference to FIGS. 8A to 8E illustrating the integral molding procedure with a silicone rubber for the preparation of a push button switch covering member by a cross section. Namely, the flowable resin composition 82 is cast from the dispenser nozzle 81 into the cavity of a lower mold 83, as is illustrated in FIG. 8A, to fill up the cavity (see FIG. 8B). After curing of the thermosetting resin composition by heating in situ in the cavity of the lower mold 83 to form a cured keytop 86, as is illustrated in FIG. 8C, a sheet of an uncured silicone rubber stock 84 is mounted to cover the cavity of the lower mold 83 and compression-molded with the upper mold 85 (see FIG. 8D) to close the metal mold into a cured silicone rubber part consisting of the base part 75, riser part 74 and keytop base 72, which is integrally bonded to the resin-made keytop 86 as is illustrated in FIG. 8E.
As a consequence of the large molding shrinkage of the resin composition 82, it is unavoidable that a large gap space 90 is formed around the cured resin part 86 with the wall of the cavity of the lower mold 83 as is illustrated in FIG. 8C so that the silicone rubber stock 84 under compression molding naturally intrudes into and fills up the gap space 90 around the cured resinous keytop 86 as is illustrated in FIG. 8D and cured therein to be bonded to the resin part 86 although the joint surface between the two parts cannot be even with irregular joint line between the parts appearing on the side wall of the keytop 86 as is illustrated in FIG. 8E. Moreover, the large difference in the molding shrinkage between the rubber part 72 and the resin part 86 may eventually cause warping or deformation of the two-parts composite body as molded.
Further, the thermosetting resin disclosed in Japanese Patent Kokai 6-60767, as compared with those disclosed in Japanese Patent Kokai 6-309988, in the course of curing rapidly loses bondability with a silicone rubber so that the integral molding process thereof with a silicone rubber part is not versatile enough. Moreover, the resin composition is not highly transparent so that the keytops molded with the resin composition cannot be brightly colored.